Description:
------------
When you use a timezone with daylight saving time (like Europe/Berlin) and the date of the system running the PHP script is in this daylight saving time (like 2012-06-15), DateTime::add() will produce results that are off by 1 hour.
When the date on the local machine is not in the daylight saving time, the same computation will yield the expceted results.
This bug also only appears, when the DateTime instance was created using a timestamp, so a workaround would be the following: Instead of
$date = new DateTime(gmmktime(0, 0, 0, 1, 1, 2012));
use
$date = new DateTime('2012-01-01');
The script was tested with the following systems, all having the same problem:
- Windows XP, PHP 5.3.10
- Windows XP, PHP 5.3.13
- Windows 7, PHP 5.3.10
- Arch Linux (x86_64), PHP 5.4.3
- Debian Squeeze, PHP 5.3.10
Test script:
---------------
<?php
$oneMonth = new DateInterval('P1M');
$date = new DateTime('@'.gmmktime(0, 0, 0, 1, 1, 2012));
echo $date->format('Y-m-d H:i:s')."\n";
$date->add($oneMonth);
echo $date->format('Y-m-d H:i:s')."\n";
Expected result:
----------------
2012-01-01 00:00:00
2012-02-01 00:00:00
Actual result:
--------------
2012-01-01 00:00:00
2012-02-01 01:00:00

This is also affecting DateTime->modify() in PHP 5.3.21.
This appears to only affect DateTime objects with timezone_type of 1, and maybe 2.
Of course, it's impossible to directly query a DateTime object's timezone_type, so you have to get the string representation from print_r and inspect that in order to code around this bug.
Unbelievable.